Optical computing devices, also commonly referred to as “opticoanalytical devices,” can be used to analyze and monitor a substance in real time. Such optical computing devices will often employ an optical element or optical processing element that optically interacts with the substance or a sample thereof to determine quantitative and/or qualitative values of one or more physical or chemical properties of the substance. The optical element may be, for example, an integrated computational element (ICE) core, also known as a multivariate optical element (MOE), which is essentially an optical interference based device that can be designed to operate over a continuum of wavelengths in the electromagnetic spectrum from the UV to mid-infrared (MIR) ranges, or any sub-set of that region. Electromagnetic radiation that optically interacts with a substance is changed and processed by the ICE core so as to be readable by a detector, such that an output of the detector can be correlated to the physical or chemical property of the substance being analyzed.
An ICE core typically includes a plurality of optical layers consisting of various materials whose index of refraction and size (e.g., thickness) may vary between each layer. An ICE core design refers to the number and thickness of the respective layers of the ICE core. The layers may be strategically deposited and sized so as to selectively pass predetermined fractions of electromagnetic radiation at different wavelengths configured to substantially mimic a regression vector corresponding to a particular physical or chemical property of interest of a substance. Accordingly, an ICE core design will exhibit a transmission function that is weighted with respect to wavelength. As a result, the output light intensity from the ICE core conveyed to a detector may be related to the physical or chemical property of interest for the substance.
After manufacture, and before being placed in downhole use, each optical computing device must be carefully calibrated against known reference fluids for temperature and pressure ranges expected to be encountered in the field. The calibrated optical computing devices are then installed as part of a downhole tool and re-tested to validate the optical responses from the optical element. In some cases, anomalous optical responses may occur upon field-testing the optical computing device. An anomalous optical response essentially consists of an optical response that is either too high or too low as compared to the calibration data. Optical response anomalies can be caused by, for example, damage to an optical element (e.g., a light focusing or collimating element), the changes in optical fastening materials under the stress of high temperatures and pressures. Optical response anomalies can arise after replacement of parts during servicing of the tool resulting in inconsistencies between optical system components and signal processing between the manufacturing calibration and tool implementation. Other sources of optical response anomalies can result from assembly and/or disassembly variations of the optical computing device in the downhole tool.